1. Field of the Invention
The present invention relates to a temperature dependent electric addressing system for a device whose operation is dependent on the local heating communicated to a thermo-active material layer by an electrode. The invention applies particularly to the case of display screens with matrix control using a liquid crystal of the smectic type and a set of conducting strips heated transitorily by Joule heating.
The electrodes causing the heating of the layer of a material such as a liquid crystal are successively traversed by a pulsed and recurrent heating current. Since the operating temperature of the device is well defined according to the material used, it is clear that the electric heating power required for reaching this temperature depends greatly on the resistance of the electrode and its variation with the temperature. Although it is relatively easy to control this power as a function of the ambient temperature, it is much more difficult to take into account the local temperatures inside the device. Depending on the type of use, considerable gradients appear from one electrode to another. So as to supply the electrode concerned with the required energy sufficient to accomplish the thermal cycle imposed by the characteristics of the thermo-active material, it is advantageous to make the device temperature dependent by taking into account the particularities of the resistance electrodes.
2. Description of the Prior Art
It is known to make such devices heat dependent by using a microprocessor. Since the current in the electrode results from the application of a voltage pulse of amplitude V and duration T, the power supplied for heating an electrode may be modulated by acting on the duration T and keeping the amplitude V fixed. The microprocessor manages a read only memory containing the networks of "pulse width as a function of the temperature" curves. The microprocessor takes into account the address of the electrode and its previous excitation conditions: number of times that the electrode has been acted on in a given period of time, time since the last action etc.
From this data it will call up from the memory the suitable pulse width. The drawback of this method is that it requires management by means of fixed data which does not easily take into account the real parameters and all the possibilities of use. In fact, the thermal state of the material in contact with an electrode is only known by data external to the electrode-material interface.